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Stock (Factory) vs. Custom Firmware

Factory firmware for ChromeOS devices is built for booting ChromeOS, of course. Some models also include the capability of booting other OSes in factory firmware, but most do not (or have serious implementation bugs).

Custom firmware adds the ability to boot GalliumOS to all models, and fixes factory bugs. Some firmware types also improve other functionality. See below for details.

Removes the ability to run ChromeOS, creating a small risk of bricking your device

Offers the best support for booting all OSes besides ChromeOS (including Windows).

On many models, adds full hardware support for virtualization (vmx / VT-x)

Fixes many bugs and/or idiosyncrasies associated with the stock firmware (but could potentially add new ones)

Essentially turns your ChromeOS device into a "regular" PC / laptop

Factory Firmware

Factory firmware capabilities vary by model family. Haswells have good support for booting GalliumOS out of the box. Broadwells should have good support, but unfortunately the factory firmware has inconvenient video bugs. Bay Trail, Braswell, and Apollo Lake models don't include any support whatsoever. Skylake models also have video bugs in factory firmware. Kaby Lake models have OK support for eMMC devices, but fatal bugs for NVMe devices.

In all cases, custom firmware is preferred. Custom RW_LEGACY firmware is available for nearly all Intel models, and is very quick, safe, and simple to update.

Firmware and NVRAM Flags

Both the crossystem and GBB flags listed below are only relevant when booting legacy mode via RW_LEGACY (CTRL+L on dev mode boot screen); they do not have any effect when using custom BOOT_STUB or full firmware.

crossystem

ChromeOS System (crossystem) flags are firmware level settings which are stored in volatile memory, and are therefore lost/reset if the battery on your Chromebook is fully drained or disconnected; they do not require firmware write protect to be disabled in order to set/unset them.

The dev_boot_legacy flag needs to be set (=1) in order to enable legacy booting (and installation/booting of GalliumOS). Most users do not need to set this manually, as it is set automatically when installing a RW_LEGACY firmware payload, but should be aware that it can be unset via battery drain/disconnect. This can be problematic if GalliumOS is the only OS installed on the system (as it would require the user to restore ChromeOS to re-enable legacy booting), and so it is recommended that users who are not dual-booting set the GBB flags below to ensure legacy boot capability is not "turned off" if battery power is lost.

Note: many other sources mistakenly insist that dev_boot_usb needs to be set as well, but it does not - that flag only enables booting ChromeOS (or an OS with a ChromeOS-signed kernel) via CTRL+U on the developer mode boot screen; it has nothing to do with booting from USB in legacy boot mode (CTRL+L).

GBB Flags

Google Binary Block (GBB) flags are firmware level settings stored directly in the firmware flash chip itself in a read-only (RO) area, and so require the firmware write protect screw to be removed before setting. The GBB flags are used to set the default boot mode (ChromeOS or legacy boot), developer mode boot screen timeout (2s or 30s), prevent exiting developer mode via spacebar, and to enable legacy booting regardless of crossystem setting, among other things.

Most users should not attempt to set these manually, but instead use MrChromebox's ChromeOS firmware utility script to set them based on their preferred default boot mode and boot timeout (the script automatically sets the dev mode and legacy boot mode overrides for all options except factory default). For those curious, a full description of all the GBB flags can be found in the Google source code here